Copper Wire Bond Research Articles

An Assessment of Risk of Fracture During Wirebond Over Active Circuits on ULK Dies

Mechanical integrity of the dielectric stack is challenged by the trend toward porous, lower dielectric constant interlayer dielectric (ILD) materials. During copper wirebonding over active circuits, the fracture of ultralow-k (ULK) dielectrics is a significant reliability challenge due to both the impact load and the high ultrasonic energy transmitted to the ILD stack. Thus, there is a need to assess the propensity of the ILD to fracture under the wirebonding process conditions. In this paper, a numerical investigation of the risk of fracture in ULK dies due to the Cu wirebonding process is presented. A recently developed computational technique for modeling cracks using behavioral approximations built directly on the geometrical model (enriched isogeometric analysis), as opposed to the analysis of a meshed geometry, is adopted to model damage in the ULK stacks. The cohesive damage induced in the ILD stack during the process steps, which is a measure of the risk of crack initiation, is used as an indicator of the reliability risk. The damage accumulation during the Cu wirebonding process is estimated to identify weak interfaces and potential sites for crack nucleation as well as damage nucleation patterns. Furthermore, the critical process condition at which crack nucleation is likely is identified by analyzing the damage induced during the impact and ultrasonic excitation stages. Also, representative ILD stack designs with varying Cu percentage are compared to assess the relative risk of fracture.

Decapsulation of Plastic Encapsulated Microelectronics with Copper Wire Bonds

Abstract A novel method for copper wire bond decapsulation was developed, which involves conducting a sequential process to ensure minimal corrosion or, in some cases, no corrosion/degradation of the copper wire. It requires a pre-treatment which involves mechanical milling or acid etching; the main decapsulation process, utilizing a mixture of nitric and sulfuric acid under electrically biased conditions; and a post decapsulation procedure of encapsulant debris removal using a solvent. Previous studies use decapsulation methods that reveal either the ball bond or the wedge bond, separately, and thus cannot be used to perform wire pull tests. Exposing both the ball and wedge bond is not trivial, as the wire tends to break at the loop due to the long exposure time. The three-step process, when properly executed, ensures that the entire wire along with both ball and wedge bonds are exposed with minimal damage. Five parameters were found to affect the decapsulation process. These are as follows: acid ratio, biasing voltage, time of exposure, temperature and mold compound type. Careful control of these parameters can result in little to no copper wire degradation.

Heavy copper wire-bonding on silicon chips with aluminum-passivated Cu bond-pads

Thick electroplated Cu bond-pads have recently been shown to allow for heavy Cu wire-bonding on silicon power devices. The Cu surface oxides present on these pads are a major concern for the bonding process and for a sufficiently stable bond formation. They currently have to be removed after the die-attach and prior to wire-bonding. To avoid such removal, the application of a thin Al coating on the Cu bond-pads is investigated for its passivating ability and its suitability for the bonding process.

Non-destructive degradation study of copper wire bond for its temperature cycling reliability evaluation

Wire bonding is essential for the electrical connection of integrated circuit (IC) devices, therefore its quality and reliability is of utmost importance. During the wire bonding process, several parameters need to be well controlled in order to achieve a well bonded wire. Furthermore, the migration to copper (Cu) wire from gold (Au) due to its high cost has resulted in an even more stringent and narrow process window. Current industrial practices to evaluate wire bond quality after the assembly and packaging process are either done destructively which may result in loss of critical information, or non-destructively which are limited by resolution, cost and time. In this work, the quality of copper wire bond is being evaluated by electrical means that is non-destructive, fast and accurate. This makes it suitable for use in the production line for wire bond quality evaluation. Experimental results showed that there is a good correlation with conventional wire assessment methods. Furthermore the electrical method is sensitive enough to pick out degraded wires that conventional methods are unable to identify.

Ensuring Suitability of Cu Wire Bonded ICs for Automotive Applications

The transition to replace gold with copper bond wires in semiconductor components, primarily driven by the ever increasing price of gold wire, has been under way for several years. Cu wire bonds (Cu-WBs) are technically more challenging than gold to produce, requiring improved designs, processes and equipment. After introduction in consumer products, their use is now migrating to automotive electronics where product integrity for quality, reliability and durability (QRD) and safety over 10–15 years in a demanding harsh environment is paramount, in addition to managing cost in the highly competitive global automotive market. Reliability issues with some Cu wire bonded components detected during the rigorous product validation durability–reliability tests of automotive electronics, however, are starting to appear. The indications are that only optimized package design with well-controlled assembly processes are suitable for high reliability (hi-rel) harsh environment applications such as automotive, military and aerospace. A concern is that non-optimized Cu-WBs and package materials issues are being detected in module-level durability validation tests in parts that were qualified as automotive grade per AEC Q-100 or AEC-Q101. This article will explore the issues and discuss potential solutions as the Automotive Electronics Council (AEC) – the organization that defines requirements for automotive grade electronic components – works to update qualification procedures for evolving Cu-wire bond technology.

Corrosion reliability of copper wirebond (CuWB) packages-Impact of voltage and corrosive ions from packaging materials

Reliability of electronic devices used in extreme and harsh conditions such as in automotive applications is often associated with prevention of corrosion at the interfaces of dissimilar interconnect metals and of metal (Al or Cu) bonding pads. Packaging materials, especially molding compounds, contain several ionic components and can uptake certain level of moisture to provide electrolytic conditions to initiate corrosion processes. Probability of occurrence of corrosion in devices with high voltage applications is of special interest as many analog devices operates at higher voltages and with new CuWB packages there is a higher susceptibility of corrosive failures. This paper examines the impact of voltage on the mold compound compatibility with CuWB packages up to 65 V. Bias HAST reliability evaluation of various mold compounds at different voltages will be described. Corrosion of bonding pad (Al) and at the Al-Cu intermetallic compound interface was observed in the presence of certain ions present in mold compounds. Even though current mold compounds are “green”, the allowable amounts of halide ions can far exceed the ppm limit to prevent CuWB corrosive failures. Bromide ions are known to be very corrosive to metals. Br ions are still present in some mold compounds. The impact of the Br ion on CuWB reliability will be described. Additive impact of Br ions in presence of Cl will be examined. Another corrosive component present in many mold compounds, sulfur compounds, also can be corrosive under certain reliability stress conditions. Difference in corrosion behavior of Cu and Pd –Cu wirebonded packages due to sulfur compounds will be presented. Corrosive impact of high levels of Cl on the Al pad corrosion leading thick aluminum oxide growth and reliability failures will be discussed.

Bondability of Second Copper Wire Bonds on Silver and Nickel-Palladium–Gold-Silver Metallization

Several types of metallization have been used in order to enhance the bondability of wire bond. The introduction of copper (Cu) wire bond further increases the requirement of finding the suitable metallization. In this paper, two types of metallization, namely silver (Ag) and nickel-palladium–gold-silver (Ni/Pd/Au/Ag), deposited on Cu substrate were used as the bond pad for the second Cu wire bond. Wire pulling test was conducted on the Cu second bond by placing the hook near the second bond in order to obtain consistence stich of Cu wire bond. The characteristic remaining area of stich was obtained using the image processing software Image J. It was observed that the pull strength of the Cu second bond was higher for Ag metallization compared with that of Ni/Pd/Au/Ag metallization. It was also shown that the Cu second bond on the Ag metallization has higher contact diameter compared with that of Ni/Pd/Au/Ag metallization. The contact diameter of the Cu second bond on Ag metallization has higher value and it can be correlated with its higher value of pull strength. The characteristic area was found to be proportionally related with the pull strength of second bond.

Reliability of copper wire bonds on a novel over-pad metallization

Wire bonding technology is used in most semiconductor products. Recently, high gold prices have forced semiconductor manufacturers to replace Au wires with Cu wires. Because Cu wire bonds are vulnerable to high temperature and humidity, they remain unpopular in automotive and industrial applications with narrow-bond-pad pitches and small deformed ball diameters. To avoid forming the corrosive Cu-rich intermetallic compound Cu9Al4, the use of a Ni/Pd(/Au) over-pad metallization (OPM) structure produced by electroless plating on the Al metallization has been proposed. However, certain technical issues must be overcome, such as variations in the purity and thickness of the plating. To tackle these issues, a novel OPM structure produced by physical vapor deposition is proposed and evaluated in this study.

Reliability of Cu Wirebonded ICs for Automotive Applications

The transition to replace gold with copper bond wires in semiconductor components, primarily driven by the ever increasing price of gold wire, has been under way for several years. Cu wire bonds (Cu-WBs) are technically more challenging than gold to produce, requiring improved designs, processes and equipment. After introduction in consumer products, their use is now migrating to automotive electronics where product integrity for quality, reliability and durability (QRD) and safety over 10–15 years in a demanding harsh environment is paramount, in addition to managing cost in the highly competitive global automotive market. Reliability issues with some Cu wire bonded components detected during the rigorous product validation durability–reliability tests of automotive electronics, however, are starting to appear. The indications are that only optimized package design with well-controlled assembly processes are suitable for high reliability (hi-rel) harsh environment applications such as automotive, military and aerospace. A concern is that non-optimized Cu-WBs and package materials issues are being detected in module-level durability validation tests in parts that were qualified as automotive grade per AEC Q-100 or AEC-Q101. This paper will explore the issues and discuss potential solutions as the Automotive Electronics Council (AEC) – the organization that defines requirements for automotive grade electronic components – works to update qualification procedures for evolving Cu-wire bond technology.

Effect of palladium on the mechanical properties of Cu–Al intermetallic compounds

There is growing interest in copper (Cu) wire bonding due to its significant cost savings over gold wire. However, concerns regarding package reliability and corrosion susceptibility have driven the industry to develop alternative materials. Recently, palladium coated copper (Pd–Cu) wire has been widely used in fine pitch applications as it is believed to improve reliability of copper wire bonds on aluminum (Al) pads. However, the effect of palladium on the mechanical properties and corrosion resistance of Cu and Cu–Al intermetallic compounds (IMCs) has not been studied in detail. In this paper, bulk alloys of Cu and Cu–Al alloys with different concentrations of Pd were studied. Among the three IMCs studied, CuAl (H∼8.5GPa) was found much harder than CuAl2 and Cu9Al4 (H∼6.0GPa). Young’s moduli of CuAl and Cu9Al4 IMCs were around 155GPa and are much stiffer than the one for CuAl2 (∼110GPa). The addition of a few percent Pd only slightly increases the hardness and Young’s modulus of the IMCs.

Microstructural investigations of aluminum and copper wire bonds

Ultrasonic wire bonding is a widely used technology to interconnect chips in the packaging industry. It is known that the bond quality depends on the microstructure prevalant in the bond. Similarly, the materials used as well as the bonding parameters depend also on the microstructure. However, the actual used materials such as aluminum and gold are either characterized by poor conductivity or high costs. Due to the outstanding properties of copper, it is a promising candidate for wire bonds. Nevertheless, a thorough investigation on the interrelationship between the material combinations, the processing parameters and the resulting microstructure for copper and aluminum wire bonds need to be conducted. Facing these unresolved issues, samples of aluminum and copper wires were extracted during various states of the bonding procedure. The samples were analyzed to gather information about microstructural influences on the bonding parameters. As a result a conclusion can be drawn regarding the ultrasonic wire bond strength/load subject to the bonding parameters. Furthermore, the different deformation behaviour of copper and aluminum will be depicted by employing the following analytical methods: hardness tests, light microscopy, and scanning electron microscopy.

A Case Study of the Reliability of Copper Bond Wires In Plastic Encapsulated Integrated Circuits

Sixteen plastic encapsulated integrated circuits were initially received as part of a root-cause failure investigation, twelve of which contained copper bond wires and the other four containing gold bond wires. It was reported that eight of the integrated circuits had failed in the field with intermittent or complete malfunction of the components. Electrical testing confirmed the reported failure and found temperature sensitive, intermittent open circuits on various pins coinciding with the sample history. C-Mode scanning acoustic microscopy (CSAM) was performed on the devices, with each sample exhibiting varying degrees of delamination between the lead-frame and the plastic encapsulant. The backside of the package was polished into the underside of the lead-frame to preclude the introduction of any contamination due to chemical decapsulation. This allowed the mechanical removal of the leads from the package and inspection of the stitch bonds. Scanning electron microscope (SEM) inspection revealed that the copper stitch bonds had corroded to some extent (ranging from mild corrosion to complete consumption of the stitch bond) in each of the examined bonds, whereas the gold stitch bonds were unaffected. Overall inspection of the leads identified chlorine contamination along the delaminated interface.

High temperature degradation of palladium coated copper bond wires

High temperature storage lifetime tests of palladium coated copper bond wires (pcc-wires) beyond 1000h@150°C lead to an increased number of broken stitches during wire bond pull test. In this article we show that there is an intrinsic degradation of pcc-wires: defects in the Pd layer allow a temperature driven diffusion of Cu to the Pd surface reacting to CuO on the wire surface. Voids in the range of several microns in the Cu wire core weaken the bond wire strength to very low values.The degradation mechanism of pcc-wires is found in both cases, in molded packages and at non-molded wires from the spool. We present results after temperature storage at 150°C, 175°C, 200°C, and 250°C up to 3000h.

Wire Bond Shear Test Simulation on Flat Surface Bond Pad

The reliability and the bond strength of wire bonding electronic packages are appraised using wire bond shear test. In this study, a three-dimensional non linear finite element model was designed to simulate the stress response of the bonded wire during wire bond shear test on a flat surface bond pad. Comparisons between three types of wire material:gold (Au), aluminium (Al) and copper (Cu) were done to scrutinize the effects of wire material on the stress response of bonded wire during wire bond shear test. The simulation results showed that copper wire bond induces higher stress compared to aluminium and gold wire bond during wire bond shear test.

Copper Wirebond Compatibility with Organic and Inorganic Ions Present in Mold Compounds

Copper wirebond (CuWB) packaging is emerging as a replacement to gold wire technology because of the increases in the price of gold. However, copper is more susceptible to corrosion than inert gold so there are several reliability concerns regarding the integrity of the Cu-Al interface. Copper wirebond compatibility with the surrounding encapsulating mold compound is critical, as this material combination can produce a corrosive environment in the presence of moisture and ionic impurities. Mold compounds consist of different chemical components that can yield several ionic organic and inorganic species. The effect of chloride on CuWB reliability has been published in the literature but the impact of other ions typically present in the mold compound matrix is not widely discussed. It is important to understand the role of these ions on the CuWB reliability to design appropriate mold compounds for product encapsulation as some of the ions are corrosive while others are benign or beneficial in nature. This paper examines the impact of organic anions such as formate, acetate, and oxalate, and inorganic anions phosphate and nitrites, on CuWB reliability with regard to the integrity of the vulnerable Cu-Al interface. Corrosion depends on the pH conditions of the mold compound matrix in the presence of moisture. Cu-Al integrity under various ionic concentrations and pH conditions will be presented and the effect of these ions on the corrosive behavior of chloride will be examined. The combined influences of different anions on the Cu-Al integrity will be discussed. Finally, a relatively simple method to examine the integrity of the Cu-Al interface will also be described.

Advanced Surface Finishes for Ceramic Electronics for Soldering, Conductive Adhesive, Aluminum- , Gold-, and Copper Wire Bonding.

Surface finishes for ceramic electronics are an important feature. . The suitable surface finish allows the appropriate assembly technology to be performed on the ceramic substrate. The paper will present the possible surface finish options for ceramic substrates and include the advantages and challenges of each surface finish. Furthermore it will show the potential of a new surface finish called “Direct EPAG”. The demand for these new finishes is based on needs for new assembly technologies like copper wire bonding and for high frequency applications. Furthermore Nickel which the main layer of many surface finishes like ENIG or ENEPIG, is more and more regarded as an unpleasant element due to several disadvantages. All these concerns are supporting the introduction of a new surface finish called “direct EPAG” (Electroless Palladium, Autocatalytic Gold) finish. In particular, its suitability for copper wire bonding and its fine line capability makes it very attractive from a cost standpoint. In addition, the mentioned technological drawbacks of nickel based finishes could be solved by applying a revolutionary surface finish: “direct palladium with an optional direct gold layer on copper”. The direct deposition of palladium on copper with an optional gold layer does have a number of technological, environmental and economic benefits. The suggested paper will describe and discuss the advantages and challenges using this new surface finish. It will also exhibit the results of various soldering and wire bond tests on the EPAG finish.

Corrosion Study and Intermetallics Formation in Gold and Copper Wire Bonding in Microelectronics Packaging

A comparison study on the reliability of gold (Au) and copper (Cu) wire bonding is conducted to determine their corrosion and oxidation behavior in different environmental conditions. The corrosion and oxidation behaviors of Au and Cu wire bonding are determined through soaking in sodium chloride (NaCl) solution and high temperature storage (HTS) at 175 °C, 200 °C and 225 °C. Galvanic corrosion is more intense in Cu wire bonding as compared to Au wire bonding in NaCl solution due to the minimal formation of intermetallics in the former. At all three HTS annealing temperatures, the rate of Cu-Al intermetallic formation is found to be three to five times slower than Au-Al intermetallics. The faster intermetallic growth rate and lower activation energy found in this work for both Au/Al and Cu/Al as compared to literature could be due to the thicker Al pad metallization which removed the rate-determining step in previous studies due to deficit in Al material.

Effect of Shielding Gases on Ball Forming and Bondability in Copper Wire Bonding

Effect of Shielding Gases on Ball Forming and Bondability in Copper Wire Bonding

Copper Wirebond Compatibility with Organic and Inorganic Ions Present in Mold Compounds

Copper wirebond (CuWB) packaging is emerging as a replacement to gold wire technology because of the increases in the price of gold. However, copper is more susceptible to corrosion than inert gold so there are several reliability concerns regarding the integrity of the Cu-Al interface. Copper wirebond compatibility with the surrounding encapsulating mold compound is critical as this material combination can produce a corrosive environment in the presence of moisture and ionic impurities. Mold compounds consist of different chemical components that can yield several ionic organic and inorganic species. The effect of chloride on CuWB reliability has been published in the literature, the impact of other ions typically present in mold compound matrix is not widely discussed. It is important to understand the role of these ions on the CuWB reliability to design appropriate mold compounds for product encapsulation as some of the ions are corrosive while others are of benign or beneficial in nature. This paper examines the impact of organic anions such as formate, acetate and oxalate and inorganic anions phosphate and nitrites on CuWB reliability with regards to the integrity of the vulnerable Cu-Al interface. Corrosion depends on the pH conditions of the mold compound matrix in presence of moisture.Cu-Al integrity under various pH conditions and ionic concentrations will be presented and the effect of these ions on the corrosive behavior of chloride will be examined. The combined influences of different anions on the Cu-Al integrity will be discussed. Finally, a relatively simple method to examine the integrity of the Cu-Al interface will also be described.

Wire Bond Shear Test Simulation on Sharp Groove Surface Bond Pad

Wire bonding process is first level interconnection technology used in the semiconductor packaging industry. The wire bond shear tests are used in the industry to examine the bond strength and reliability of the bonded wires. Hence, in this study thesimulation on wire bond shear test is performed on a sharp groove surface bond pad. ANSYS ver 11 was used to perform the simulation. The stress response of the bonded wires are investigated.The effects of three wire materials gold(Au), aluminum(Al) and copper(Cu) on the stress response during shear test were examined. The simulation results showed that copper wire bond induces highest stress and gold wire exhibits the least stress response.